1. Field of the Invention
The invention is related to a downhole milling and drilling assembly, more particularly to a whipstock assembly having a shearable connection with enhanced shear strength in one direction.
2. Background of the Related Art
In the drilling of oil and gas wells, lateral wellbores are often required to form another wellbore into an adjacent formation, to provide a perforated production zone at a desired level, to provide cement bonding between a small diameter casing and the adjacent formation, or to remove a loose joint of surface pipe. To create the lateral wellbore, milling tools are used for removing a section or a xe2x80x9cwindowxe2x80x9d of existing casing from a primary wellbore. The milling tools have cutting blades and typically utilize a diverter such as a whipstock to cause the tool to be moved laterally while it is being moved downwardly and rotating in the wellbore to cut an angled opening, pocket or window in the well casing or a borehole.
Formation of a lateral wellbore is typically performed in a step saving manner according to the following steps: An anchoring member or packer is set in a wellbore at a desired location below the location where the lateral wellbore will be formed. The packer acts as an anchor against which tools above it may be fixed in place in the wellbore. The packer typically has a key or other orientation indicating member and the packer""s orientation is checked by running a tool such as a gyroscope indicator into the wellbore. A whipstock/cutter combination tool is then run into the wellbore and landed in the packer whereby the whipstock is oriented in the direction of the desired lateral wellbore. The cutter is connected to the whipstock by a shearable member, like a bolt. In this manner, the cutter and whipstock can be run-in to the well together, saving an additional trip. Pushing on the cutter shears the bolt, freeing the cutter from the tool. Rotation of the string and the cutter can then begin the formation of the lateral wellbore.
Multiple lateral wellbores in a well necessitate the setting of a whipstock at various vertical locations in the wellbore. Rather than removing and relocating the packer, extensions are used between the whipstock and the packer to accurately locate the whipstock at that point in the wellbore where the next lateral wellbore will be formed. Depending upon the distance between the packer and the new wellbore, an extension member can add significant weight to the combination tool. In some instances, the weight of the whipstock, stinger, extensions and accessories can exceed the shear strength of the connection member between the cutter and the whipstock, which is designed to shear only upon the placement of weight on the connection from above. For example, in a 5xc2xdxe2x80x3 wellbore, a whipstock and stinger typically weighs around 1,000 lbs. and the shear value of the shearable connection between the whipstock and cutter is about 16,000 lbs. An extension and accessories, like a stabilizer, could add 16,000 lbs. to the assembly bringing the weight near the shear value of the connection between the whipstock and cutter. In another example, a 9⅝xe2x80x3 wellbore typically utilizes a whipstock and stinger having a combined weight of 3,000 lbs. The shear value of the connection between the whipstock and cutter in these wells is around 30,000 lbs. Extensions and accessories for a lateral wellbore can weigh as much as 30,000 lbs., bringing the total weight of the assembly over the shear value of the connection. A failure of the shearable connection from tensile force placed upon it from below could result in a loss of the whipstock assembly and/or the packer therebelow and damage to the well. Simply increasing the shear strength of the connection member is not a viable option, since compressive force from above to shear the strengthened connection may not be available, and damage to parts of the assembly may result from the increased force.
In addition to the need for enhanced tensile resistance to the shearable connection between the whipstock cutter, there are instances when increased compressive shear strength is needed to prevent a failure of the connection when the assembly is being pushed into a horizontal wellbore against its own weight and friction with the wellbore casing.
There is a need therefore for a whipstock assembly with a shearable connection between the cutter and whipstock that can withstand tensile forces applied by the weight of the whipstock assembly. There is also a need therefore for a shearable connection between a whipstock and a cutter which will tolerate greater forces in one direction than in an opposite direction but still fail upon the application of a compressive force from above. There is a further need therefore, for a shearable connection member which has greater strength in tension than in compression.
The present invention discloses a whipstock assembly for use in a wellbore to form a lateral wellbore therefrom. In one aspect, a whipstock is attached to a cutting tool by a shearable connection whereby the whipstock and cutting tool assembly may be run into the wellbore simultaneously. Upon compressive force from above, the shearable connection fails and the cutting action can begin. The shearable connection is designed to fail in compression but to withstand forces in tension brought about by the whipstock, accessories and extensions required to properly place the whipstock above a preset packer in the wellbore. In one aspect, the shearable connection means provides a first set of shearable members with equal shear resistance to tensile and compressive forces applied between the whipstock and cutter. Another set of shearable members provide shear resistance against tensile forces between the whipstock and cutter but do not provide shear resistance against compressive forces. The resulting connection is stronger in tension than in compression and failure of the connection due to the weight of the whipstock assembly is less likely. In another aspect of the invention, a retractable finger provides additional shear strength in tension. The retractable finger is spring-loaded and is housed in a slot formed in a lug portion of the whipstock. When the shearable connection is in tension, the finger interferes with a surface formed in the cutter, adding additional shear strength to the connection. When the shearable connection is in compression, the finger folds into the slot, providing no additional resistance against the compressive force. In another aspect of the invention the shearable connection is designed to provide additional shear resistance to compression forces but not to tensile forces applied between the whipstock and cutter.